Plutonium is an important transuranic, radioactive, fissile element. It serves a number of functions, ranging from power production in pacemakers to use in weaponry. Because of the value of plutonium and its potential hazard, the amount of plutonium processed and handled must be known accurately. Determining how much plutonium is present in its various forms is referred to as plutonium accountability. As the amounts of plutonium in a process are measured, the measurements will suggest that some plutonium is "lost"; that is, the sum of all the measured quantities of plutonium in a process will be less than the total amount that was initially present. The material is "lost" in the sense that it is not known if (1) the material is present but, because of measurement uncertainty and conservatism, verification of the presence of all the plutonium is not possible; (2) the plutonium has become part of the process equipment by adhering to surfaces; or (3) the material has somehow gotten away from the process facility, for example, by a few micrograms having adhered to protective clothing. Furthermore, because the quantities of plutonium handled are generally small, on the order of grams, recovery of even trace quantities can be important in knowing exactly how much plutonium is in different stages of a process and how much has been "lost".
A currently-used process of plutonium recovery is applied to two particular low-level waste categories: LECO crucibles and incinerator ash. These materials are a particular problem because they are porous. To recover trace plutonium, the crucibles and ash are fused with ammonium bifluoride in a low temperature reaction then the fused products are boiled in nitric acid. The dissolved crucible metal is then separated by centrifuging. A little more than sixty percent of the plutonium in the crucibles and greater than ninety-five percent of the plutonium in the ash are dissolved in these first two steps, which take approximately four hours. Repeating the steps provides some additional recovery.
Trace quantities of plutonium present in materials, and indeed many other hazardous metals besides plutonium, complicate disposal of those materials. Various regulations apply to disposal that generally impose controls, with attendant costs, to assure that the hazards and the human and natural environments are kept apart. Removal and recovery of these metals allows the remainder of the material to be disposed of according to somewhat less stringent requirements, thereby reducing disposal costs.
Microwaves have been applied to recover metals from porous material by dissolution in the presence of strong acids, as described in "Applications of Microwave Oven Sample Dissolution in Analysis," by R. A. Nadkarni, Anal. Chem. 1984, 56, 2233-2237. However, application of microwaves to recover plutonium or other transuranics is not reported by him. Furthermore, Nadkarni applied microwaves to coal without recovering any uranium or thorium, known to exist as trace elements in coal.
There is a need for a rapid, less expensive method for recovering metals from porous materials.